The outlines of the cranial bones can only be seen in a young skull. The single condyle is made up, as in some Reptiles, by the basi- and ex-occipitals. The parietals are short but wide. Two membrane bones - the basi-temporals - parts of the parasphenoid, underlie the base of the skull, and the rostrum, or anterior part of the parasphenoid, similarly underlies the mesethmoidal septum. The palatine bones extend forwards to the maxilla and articulate behind with the rostrum. The pterygoids articulate in front with the palatines and the basipterygoid processes of the rostrum, while behind they diverge and articulate with the quadrate. The maxillae have short palatal plates - 'maxillo-palatine processes' - which extend inwards above the anterior end of the palatines. The squamosal and jugal are only connected by ligament, not by bone, as in Mammalia.
The Pigeon and Sandgrouse have no vomer, the Fowl has a pointed vomer at the anterior end of the rostrum. The internal nares open between the rostrum and the palatines, and there is no hard palate. The Pigeon, like many birds, is in a permanent state of cleft palate (schizognathism), owing to the palatal plates of the praemaxilla and maxilla not meeting in the middle line.
The lower jaw of the young Fowl has five bones in each ramus, a dentary, splenial, angular and surangular with one cartilage bone, the articular. Some Birds add a coronoid, thus attaining the standard of the Lacertilian. The symphysis in the Cretaceous toothed birds was ligamentous.
The hyoid is characteristic. There are three median bones, one tongue-shaped, formed by the union of the ceratohyals, followed by the basihyal and a basibranchial. The first branchial arch is well developed, and consists of an upper epi- and a lower cerato-branchial. The joints between the several parts are synovial.
Archaeopteryx (Jurassic), Ichthyornis and Apatornis (Cretaceous) had amphi-caelous vertebrae like the Geckoes and Hatteria among living Lizards. The third cervical of Ichthyornis shows transitional characters to the modern Bird, and closely resembles the corresponding vertebrae in the Tern (Marsh). A transitory amphi-caelous stage exists in the chick on the seventh day. Some of the dorsals in the Penguin, Auks, Plovers have spheroidal faces and are opisthocaelous. The cervical ribs remain distinct for a long time in Ratitae. The division between cervical and dorsal vertebrae is somewhat arbitrary. The late Professor Rolleston considered the two vertebrae with ribs not touching the sternum as dorsals, because the ribs indent the lungs, and the last pair carries uncinate processes. Professor Huxley considers them as cervicals because the ribs do not touch the sternum. But there is embryological evidence in favour of the former view. It has been shown by Miss B. Lindsay that two anterior ribs in the Fowl and one in the Gannet are continuous at an early stage with the sternum, but become separated from it subsequently by atrophy.
The identification of two vertebrae as 'sacral' and as homologous with the vertebrae so named in Lizards depends on the following points: (1) their predominant size in the embryo; (2) the presence of free ribs ossifying by separate centres in the embryo while the preceding vertebrae are devoid of them; (3) that these ribs expand and fuse distally, as in the Crocodile; (4) that they are in relation with the acetabulum; and (5) that the nerve passing out between the ribs is the last and weakest factor in the plexus ischiadicus, as in Lizards.
Anchylosis of three dorsal vertebrae is characteristic of the Peristeromorphae (Pigeon group), while four are similarly anchylosed in the Alectoromorphae (Fowl group), but the first is a cervical.
Archaeopteryx has but five anchylosed sacral vertebrae, and the tail contains twenty vertebrae, of which the last fifteen are devoid of transverse processes, and carry each a pair of large feathers. Hesperornis has fourteen sacral and twelve caudal vertebrae, of which the last six or seven are anchylosed by their centra only, and are in other respects free.
The sternum is formed from right and left plates of cartilage, constituted by the fusion of the ventral ends of the ribs. The absence of transverse segmentation universal in Mammalia is characteristic of Birds and Reptiles. The carina, according to Gotte and Hoffmann, is formed from a single or primitively double band of tissue continuous with the clavicles. It therefore represents in part the interclavicle of a Reptile. The anterior extremity of the band forms the hypo-cleidium and the interclavicular or sterno-clavicular ligament. Their view has been recently controverted by Miss B. Lindsay, who contends that the Avian sternum consists (1) of a costal sternum derived from the ribs; (2) of a meta-sternum, apparently a growth from (1); (3) of anterior lateral processes (costal processes), either outgrowths of (1) e.g. Struthio, or formed from anterior ribs, e.g. Chick; (4) of a keel, an outgrowth of (2); and (5) posterior lateral (i.e. xiphisternal) processes derived from (2) also; together with other structures sometimes present.
The subject requires re-investigation. The shape, disposition, etc. of the several parts of the sternum vary much in Birds.
The scapula and coracoid are fused in Ratitae and connected by ligament in Carinatae. Each ossifies from a single centre. With rare exceptions (Psophia among Carinatae, Apteryx among Ratitae), the inner or anterior border of the coracoid becomes partially ligamentous. The sub-scapular process when large, e. g. Gull, Eagle, is pierced by a foramen; when small, this foramen lies in the ligament. A foramen similarly placed exists in the complete coracoid of Psophia and Apteryx, and in the Crocodile and most Lizards. It transmits a nerve for a muscle, the second pectoral in the Bird. These facts point to a homology, as maintained by Sabatier, of the sub-scapular process of the coracoid with the praecoracoid of the Reptile, i. e. with its proximal extremity. In the embryo, as figured by Miss Lindsay, there appears to be a large praecoracoid rudiment. The ligamentous portion of the coracoid is well characterized by its tough nature and parallel fibrillation. It is invaded more or less by ossification in Ratitae, and in an old Ostrich its anterior margin ossifies, inclosing a coracoid fontanelle. In a Carinate it forms part of the coraco-clavicular ligament.